Particularly in serial production, use is made of conveyor means for conveying workpieces between individual workstations where the workpieces will be e.g. assembled or processed. To convey the workpieces, the workpieces are arranged on workpiece carriers, and the workpiece carriers together with the workpieces are moved with the aid of suitable conveyor means. For moving the workpiece carriers, it is known to use friction conveyors. In friction conveyors, the workpiece carriers are not connected tightly to conveyor means but are loosely resting on rolls, for instance. The movement of the workpiece carriers is effected with the aid of the friction forces occurring between the rolls, belts or plates and friction faces of the workpiece carriers. Friction conveyors offer the particular advantage of allowing the workpiece carriers to be held up in line behind each other along the conveying path. During such a hold-up process, the corresponding drive means of the friction conveyors will merely perform a slipping movement. Thus, in friction conveyors, a bothersome and complicated disengagement from a conveyor means, e.g. from a chain, is not required.
Further, friction conveyors offer the advantage that curves and preferably branches can be negotiated in a simple manner. With the aid of branch devices, such as e.g. switches, individual workpiece carriers can be caused to merge into the conveying line or to leave the same. In this regard, friction conveyors have a high flexibility. A friction conveyor of this type is described in DE 40 36 214.
In friction conveyors, since the workpiece carriers have to be stopped at the individual work stations by means of a hold-up process, the driven rolls or conveying elements have to be provided with slip clutches. Such slip clutches are expensive and will require intensive maintenance. Further, slip clutches have to be adjusted and set, respectively, in order to define a rotational moment from which onward the slip clutch will slip. Due to wear, the setting has to be checked at regular intervals.
Known from DE 105 39 844 C2 is a friction conveyor system wherein, for accelerating workpiece carriers, individual driven conveyor elements such as e.g. rolls are provided with disk-shaped transmission elements. The disk-shaped transmission elements, which can be formed with a toothing thereon, are configured to engage a slot extending in the longitudinal direction of the workpiece carrier and comprising e.g. a tooth rack. Using such additional transmission elements, the workpiece carrier can be accelerated.
It is object of the invention to provide a friction conveyor system wherein the stopping or hold-up of workpiece carriers is simplified. A further object of the invention resides in providing a corresponding workpiece carrier for friction conveyor systems.
According to the present invention, the above objects are achieved by a friction conveyor system as defined in claim 1 and a workpiece carrier as defined in claim 2.